Three-color scan lasers are used in high speed digital photofinishing machines that scan images into memory, allow image manipulation, and finally produce hard copy printouts on photosensitive receiver material. FIG. 1 shows a system 10 having three laser sources 12, 13, and 14 that produce red, green, and blue light, respectively. The laser beams pass through respective acousto-optic modulators 16, 17, and 18 that are controlled by a pixel data stream received from printer electronics 20 synchronized by pixel clock signals 22. Laser light beams from the three acousto-optic modulators are optically combined at 24, focused on rotating polygon mirror 26, and swept across a web of receiver material 28 through an f-.theta. lens 30 to create a print line.
In the field of scan imaging systems, it is often necessary to synchronize the pixel clock to a spatial point relative to the scan line. Typically, this is accomplished by a "start-of-line" signal which is generated by the scanning beam itself passing by a fixed detector. Thus, an accurate relationship between the data path function of time and the physical scan function of space is provided once each scan line. There are several known ways to construct an oscillator which can be synchronized to a start-of-scan signal. Historically, a multiphase crystal oscillator is used because of the inherent frequency stability of crystal oscillators.
Because f-.theta. lens 30 effects different wavelengths of light differently, the apparent spatial velocity across the receiver material 28 of each of the three laser beams from acousto-optic modulators 16-18 is different from the apparent spatial velocity of the other beams. Further, the velocity of each beam is not constant over the entire sweep, but rather, follows a contouring effect profile. If the pixel clock were constant, the velocity change with position across the sweep will produce different size pixels across the sweep as illustrated in FIG. 2. Differences in apparent spatial velocity across the sweep may be as great as 4% or 5%.
Attempts have been made to compensate for differences in the apparent spatial velocity between beams by adjusting the output rate of crystal oscillators. However, crystal oscillators are very frequency stable relative to time and temperature, they are unsuited for applications wherein the frequency must be varied more than about 1/2%. This is of course insufficient to compensate for the differences in sweep velocity of 4% to 5% described above.
Oscillators having stabilized frequencies have been produced using phase-lock-loops which comprise a reference oscillator, a voltage-controlled variable frequency oscillator, a frequency divider for adjusting the frequency of the output of the variable frequency oscillator to substantially the same frequency as the output of the reference oscillator, and a phase comparator for comparing the phase of the output of the reference oscillator to the phase of the variable frequency oscillator. The variable frequency oscillator is controlled in response to the compared output. When the frequency divider or multiplier is fixed, the frequency of the variable frequency oscillator is locked, and a signal having that desired frequency can be reliably obtained.
In such prior art locked oscillators, it is possible to change the frequency to which the variable frequency oscillator is to be locked by changing the dividing rate of the frequency divider. However, if the desired frequency to which the variable frequency oscillator is to be locked is relatively great as compared with the output frequency of the reference oscillator, so that the output frequency of the variable frequency oscillator must be divided by a great number, the response time of the frequency divider becomes un-desirably long. The long response time of the frequency divider causes instability in the frequency locking of the variable frequency oscillator, and the large frequency dividing rate requires a complicated and expensive frequency divider.